1. Field of the Invention
The present invention relates generally to light emitting devices, and more particularly to an improved vertical cavity light emitting device having an improved intra-cavity aperture structure formed by selective oxidation in which the extent of the selective oxidation is controlled by impurity induced intermixing.
2. Description of the Prior Art
Vertical-Cavity Surface-Emitting Lasers (VCSELs), Surface Emitting Lasers (SELs) or Light Emitting Diodes (LEDs) are becoming increasingly important for a wide variety of applications including optical interconnection of integrated circuits, optical computing systems, optical recording and readout systems, and telecommunications. Vertically emitting devices have many advantages over edge-emitting devices, including the possibility for wafer scale fabrication and testing, and the possibility of forming two-dimensional arrays of the vertically emitting devices. The circular nature of the light output beams from these devices also make them ideally suited for coupling into optical fibers as in optical interconnects or other optical systems for integrated circuits and other applications.
VCSELs whose current flow is controlled by lateral oxidation processes show the best performances of any VCSELs in terms of low threshold current and high efficiency. In oxidized VCSELs the oxidation occurs in the lateral direction from the sides of etched mesas in the VCSEL wafers, typically under the conditions of 425.degree. C. temperature with high water-vapor content. VCSELs or any other vertical light emitting devices employing laterally oxidized layers have been typically limited to structures that have been grown upon gallium arsenide (GaAs) substrates. For further details, see U.S. Pat. No. 5,493,577, by Choquette et al.
Another advantageous feature is to have one or both mirrors in which some of the layers are laterally oxidized layers. Such mirrors achieve very high reflectivities with a relatively small number of layers compared to all-semiconductor mirrors. For example, a 99.95% reflecting bottom mirror may be grown with 5 to 7 periods instead of the more usual 25 to 30 periods. Such mirrors have desirable features over the all-semiconductor mirrors, especially for longer wavelengths, such as wavelengths greater than 1.2 .mu.m.
Since VCSELs are presently the subject of intense research and development, a great deal of results and advancements are published monthly. The following is a summary of the prior art documents which are most relevant to the problem of forming oxide apertures or regions.
Most reports of the oxidation process describe oxidation in layers of aluminum arsenide (AlAs) or aluminum gallium arsenide (Al.sub.x Ga.sub.l-x As) where the Al concentration, x, is close to unity. As reported by Choquette, et al. in "Low threshold Voltage Vertical-Cavity Lasers Fabricated by Selective Oxidation," which appeared in Electronics Letters, volume 24, pp. 2043-2044, 1994, reducing the Al concentration from x=1.0 to x=0.96 reduces the oxidation rate by more than one order of magnitude. At x=0.87, the oxidation rate is reduced by two orders of magnitude compared to x=1.0. Due to the extreme sensitivity of the oxidation rate to the Al concentration and the fact that Al concentration may vary from wafer to wafer or even over the area of a single wafer, the manufacturability of oxidized VCSELs has been questioned. In the publication by Choquette et al., entitled "Fabrication and Performance of Selectively Oxidized Vertical-Cavity Lasers," which appeared in IEEE Photonics Technology Letters, vol. 7, pp. 1237-1239, (November, 1995), this problem was noted followed by the observation that "Therefore, stringent compositional control may be necessary for wafer scale manufacture of uniformly sized oxide apertures."
A limited form of lateral control of oxidation is reported in the publication by Dallesasse, et al. entitled "Hydrolyzation Oxidation of Al.sub.x Ga.sub.l-x As-AlAs-GaAs Quantum Well Heterostructures and Superlattices," which appeared in Applied Physics Letters, volume 57, pp. 2844-2846, 1990. The same work is also described in U.S. Pat. No.'s 5,262,360 and 5,373,522, both by Holonyak and Dallesasse. In that work, GaAs-AlAs superlattices were interdiffused in selected regions by impurity-induced layer disordering (IILD). The interdiffusion was essentially complete in the selected regions, thus the interdiffused regions comprised an AlGaAs compound having an Al concentration being approximately uniform and equal to the average Al concentration of the original constituent AlAs and GaAs layers. The oxidation proceeded through the superlattice regions but not significantly into the interdiffused regions. The superlattice was not doped and contained no other structure from which to fabricate any electronic or optoelectronic device. No attempt was made to form any kind of conductive aperture or boundary or to limit the vertical extent to which the interdiffusion occured.
The oxidation rate of materials such as AlGaAs is a sensitive function of the Al concentration as described by Choquette et al. in Electronics Letters 30, pp. 2043-2044 (1994). It is therefore possible to control the extent of oxidation for multiple layers in a single process by having the layers be of different material compositions. It has also been found however, that the precise composition of a pre-oxidized layer may have a profound Iti effect on the reliability of the oxidized structure. For example, oxidized Al.sub.0.98 Ga.sub.0.02 As layers appear to be much more reliable than oxidized AlAs layers. Thus, it is preferred that all oxidizable layers in the structure have nominally the same material composition.
A thorough discussion on how the oxidation rate varies with temperature is described by Ochiai et al. in Applied Physics Letters, vol. 68, pp. 1898-1900 (1996). The authors show that for low oxidation temperatures and small oxidation depths, the oxidation depth varies linearly with time. For higher oxidation temperatures and/or large oxidation depths, the depth varies as the square root of time.